As integrated circuit component dimensions become smaller, physical wetting by etchant solutions on a substrate becomes more difficult. This is especially important for those buffered etchant solutions used in silicon dioxide etching, which in the absence of surfactants exhibit unacceptably high surface tension.
Due to the increasingly stringent requirements suitable silicon dioxide etchants must possess lower surface tension wherein the surfactant incorporated maintains an acceptable level of surface activity in hydrofluoric acid and ammonium fluoride etch systems.
Prior attempts have been made to produce stable aqueous solutions of surfactant containing ammonium fluoride and hydrofluoric acid etchants. For example, in U.S. Pat. No. 4,517,106 a fluoroalkylsulfonate is used to improve the wetting properties of the acid solutions. In U.S. Pat. No. 4,761,245 a nonionic alkylphenol polyglycidol is used for the same purpose.
In U.S. Pat. No. 4,795,582, it is disclosed that surfactants selected from the group consisting of aliphatic carboxylic acids and salts thereof, aliphatic amines and aliphatic alcohols are added to aqueous mixtures of ammonium fluoride and hydrofluoric acid to improve surface wetting. Among the various surfactant compounds, octylamine is disclosed in this patent as being suitable.
In U.S. Pat. No. 5,496,485 the patentee discloses the utilization of branched aliphatic amine surfactants. The addition of relatively substantial quantities of the surfactant, i.e., amounts in excess of 100 ppm based on the weight of etchants, are functionally required according to that patent.
The present invention provides the means to produce significantly improved surfactants which are superior from a practical standpoint of filterability, effectiveness and reduction of the surfactant quantity required.
In accordance with the invention, filtration of the surfactant must be conducted so as to reduce to a minimum the presence of particulates which may adversely affect the end use of semi-conductor components processed with compositions containing the surfactant.
In general, in pilot plant runs, integration of the surfactant into the etchant solution is effected by using a rotating mixer with a mixing period of the order of about 3 to 5 minutes. Also, in pilot plant runs, to assure a reasonable period for maximum removal of undesirable particulates from the surfactant containing etchant solution (water being the solvent unless otherwise indicated), a recirculating filtration process for a period of the order of at least about 12 hours is employed. The normal flow rate through the centrifugal pump associated with the recirculating filtration system is approximately one gallon per minute.
In commercial runs, i.e., in 100 lbs. to 3,000 lbs. batches, for example, a rotating mixer of a kind employed in the art is used to blend the surfactant into the etchant solution using a minimum blending period of approximately 3 to 5 minutes. The blend is then subjected to suitable multipass filtration operation that is continued until the desired minimum level or elimination of undesirable particulates is attained while still retaining a minimum but sufficient quantity of surfactant to provide the desired surface tension reduction in the etchant composition.
It is apparent that the quantity of surfactant retained in the etchant, for practical reasons, is the minimum amount that provides the suitable surface tension reduction. Generally, it is desired that a surfactant amount impart a functional level to the solution after the surfactant containing solution is filtered. For example, for etchants, an effective amount of surfactant retained in the filtered etchant may vary from about 10 ppm to about 100 ppm with about 35 to about 65 ppm (based on the weight of the etchant) being the more optimal quantities.